My previous article, How silicon cells generate electricity?, gives you an idea about the working and the functioning of the silicon cells. In this article, we will learn the effect of the wavelength on the working of the solar cells. Any change in the value of wavelength, changes the amount of energy falling on the surface of the silicon solar cells, hence affects their performance

But before that we need understand the basic characteristics & definitions of light.A good article for the beginners:Basics of Solar Electricity

(1) What is the wavelength of light?

Wavelength is a measure of the distance between the 2 consecutive peaks (crests) or two consecutive valleys (trough) or two 2 consecutive zero crossings. It is generally designated by Greek letter lambda and is measured in nano-meters (nm).

(2) The Dual nature of light

The light has dual nature, sometimes it behaves like a particle called photons and other times it acts as a wave. The phenomenon of traveling in straight line and the phenomenon of reflection is explained by its particle nature while the phenomenon of diffraction is explained by its wave nature. The light is the form of electromagnetic energy and it travels in the form of waves. The electromagnetic waves are different from each other in terms of their lengths called "wavelengths".All electromagnetic radiations are light but our eyes are sensitive and can see only the small portion of light called the visible light. The different type of electromagnetic radiations that reaches the surface of earth is:(i) Infrared (710 nm to 1500 nm) : These electromagnetic radiations are invisible to the human eyes but one can feel it in the form of heat.(ii) Visible light (400 nm to 700 nm) : This visible light consists of 7 colors namely, (VIBGYOR)

Violet

Indigo

Blue

Green

Yellow

Orange

Red

These colors are nothing but the electromagnetic radiations associated with different amount of energy and the wavelength.(iii) Part of Ultraviolet radiations (more harmful ultraviolet radiations are absorbed by the ozone layer) (290 nm to 400 nm) : These electromagnetic radiations can neither be seen like visible light nor be felt like Infra red radiations. These are the good source of Vitamin-D. However, prolong exposure to these radiations causes sun burn, skin damage which may lead to skin cancer and eye damage.\((1 nm = 10^{-9} meters)\)This classification of the electromagnetic radiations is based on the basis of their different wavelengths and hence different energy levels.Interesting Article: "13 interesting facts about the Sun"

(3) Understanding the relationship between the wavelength of light and the energy of photons

There is inverse relationship between the wavelength of light and the energy associated with it. That is,

When the wavelength of the light increases its energy decreases

When the wavelength of light decreases its energy increases.

The energy of the photons is generally measured in electron Volt or eV. One electron volt is the amount energy gained or lost by the electron when it is accelerated to the potential difference of 1 volt.

Where:"h" is a Planck constant with value of \(6.626 * 10^{-25} Joules-sec\)"c" is the velocity of light having value of \(3 * 10^8 m/sec\)If one knows the wavelength of light, she/he can easily calculate the energy associated with that wavelength.

The following table shows the energy associated with each wavelength of light in eV.

S.No

Electromagnetic radiations

Wavelength (nm)

Energy (eV)

1

Ultra Violet

290 - 400

4.27 - 3.099

2

Violet

400

3.1

3

Indigo

445

2.78

4

Blue

475

2.61

5

Green

510

2.43

6

Yellow

570

2.17

7

Orange

590

2.1

8

Red

650

1.9

9

Infra red

710 - 1500

1.75 - 0.826

You can see from the table above that as the wavelength of light is increasing, the energy associated with it is decreasing.

(4) What is the minimum energy required by the silicon solar cells to perform?

A typical solar cell requires about 1.1 eV (The band gap of silicon is 1.1 eV. It is the energy required by the electron to jump from valence band to the conduction band) of energy for its electrons to flow, cross the band and start the conduction of the electricity. You can see in the table above that this amount of energy is present within the infrared region of the light with wavelength of around 1127 nm. We can say in other words that the infra red region of light with wavelength <= 1127 nm (Energy is >= 1.1 eV) is capable enough to make silicon cells to start conducting and generating electricity. Obviously, the shorter wavelengths (< 1127 nm) carry higher energy is capable of extracting the electron form the valence band to the conduction band. But higher energy does not necessarily mean the increase in the amount of the current flow. Generally, the energy greater than 1.1 eV is passed out as heat in the solar panels.

(5) The efficiency of the solar cells

All of the energy that a solar cell or panel receives is not totally converted into electricity but most of it gets dissipated & wasted in the form of heat. This is because of the fact that the very specific amount of energy is required to extract the electron from the shell of the silicon atom. Therefore, any energy above 1.1 eV most of the time is not converted into electricity rather dissipated as heat into the surroundings. And because of this very fact the efficiency of solar cells are in between 15% -18% that is they are able to convert only that percentage of light energy received by them into electricity.

Conclusion

It can be said that if the wavelength of the light is > 1127nm, then its energy is not sufficient enough to extract electrons from the shell of the silicon atom. Hence there will be no flow of the electrons.However, if the wavelength of the light falling of the silicon cells < 1127 nm, then it has requisite amount of energy to knock electrons from the shell of the silicon atom and the flow of current starts.

One can summarize the above findings as below:

"If wavelength > 1127 nm, then Energy < 1.1 eV and there is no flow of current

If wavelength < 1127 nm, then Energy > 1.1 eV and the flow of current starts"

However, it is not necessary that with the increase in the energy, falling on the silicon cells, there will be increase in the current flow. The extra energy > 1.1 eV is generally dissipated as heat into the surroundings. Therefore, one cannot say that there is linear relation between energy and the flow of the current.It is true that silicon solar cells have efficiency in between 15% - 18% but by looking at their low cost, it is quite acceptable. Moreover, most of the daily electricity needs can be fulfilled with this amount of efficiency of the silicon solar cells.There is almost continuous decline in the prices of silicon PV cells from $ 76 per watt in 1977 to $ 0.3 per watt in the year 2015, an impressive CAGR (Compound Annual Growth Rate) of around 15.7%.Please look at the chart below:

By looking at the fall in the prices of the silicon cells and the technological developments in this field, one can assume that the overall efficiency of the solar roof will increase & these will become a lucrative energy options for producing electricity for the daily use.